This invention relates to a bipolar electrosurgical cutting device such as a scalpel blade, and to an electrosurgical system comprising an electrosurgical generator and a bipolar electrosurgical cutting device. Such systems are commonly used for the cutting of tissue in surgical intervention, most commonly in xe2x80x9ckeyholexe2x80x9d or minimally invasive surgery, but also in xe2x80x9copenxe2x80x9d surgery.
Electrosurgical cutting devices generally fall into two categories, monopolar and bipolar. In a monopolar device, a radio frequency signal is supplied to an active electrode which is used to cut tissue at the target site, an electrical circuit being completed by a grounding pad which is generally a large area pad attached to the patient at a location remote from the target site. In contrast, in a bipolar arrangement, both an active and a return electrode are present on the cutting device, and the current flows from the active electrode to the return electrode, often by way of an arc formed therebetween.
An early example of a bipolar RF cutting device is described in U.S. Pat. No. 4,706,667 issued to Roos, in which the return or xe2x80x9cneutralxe2x80x9d electrode is set back from the active electrode. Details for the areas of the cutting and neutral electrodes are given, and the neutral electrode is said to be perpendicularly spaced from the active electrode by between 5 and 15 mm. In a series of patents including U.S. Pat. Nos. 3,970,088, 3,987,795 and 4,043,342, Morrison describes a cutting/coagulation device which has xe2x80x9csesquipolarxe2x80x9d electrode structures. These devices are said to be a cross between monopolar and bipolar devices, with return electrodes which are carried on the cutting instrument, but which are preferably between 3 and 50 times larger in area than the cutting electrode. In one example (U.S. Pat. No. 3,970,088), the active electrode is covered with a porous, electrically-insulating layer, separating the active electrode from the tissue to be treated and causing arcing between the electrode and the tissue. The insulating layer is said to be between 0.125 and 0.25 mm (0.005 and 0.01 inches) in thickness.
In another series of patents (including U.S. Pat. Nos. 4,674,498, 4,850,353, 4,862,890 and 4,958,539), Stasz proposed a variety of cutting blade designs. These were designed with relatively small gaps between two electrodes such that arcing would occur therebetween when an RF signal was applied to the blade, the arcing causing the cutting of the tissue. Because arcing was designed to occur between the electrodes, the typical thickness for the insulating material separating the electrodes was between 0.025 and 0.075 mm (0.001 and 0.003 inches).
The present invention seeks to provide a bipolar cutting blade which is an improvement over the prior art.
Accordingly, there is provided an electrosurgical system comprising a bipolar cutting blade, a handpiece to which the cutting blade is secured, and an electrosurgical generator for supplying a radio frequency voltage signal to the cutting blade, the cutting blade comprising first and second electrodes, and an electrical insulator spacing apart the electrodes, the spacing being between 0.25 mm and 3.0 mm, and the electrosurgical generator being adapted to supply a radio frequency voltage signal to the cutting blade which has a substantially constant peak voltage value, the relationship between the peak voltage value and the spacing between the electrodes being such that the electric field intensity between the electrodes is between 0.1 volts/xcexcm and 2.0 volts/xcexcm, the first electrode having a characteristic which is dissimilar from that of the second electrode such that the first electrode is encouraged to become an active electrode and the second electrode is encouraged to become a return electrode.
The term xe2x80x9cbladexe2x80x9d is herein meant to include all devices which are designed such that both the active cutting electrode and the return electrode are designed to enter the incision made by the instrument. It is not necessary that the cutting device is only capable of making an axial incision, and indeed it will be shown below that embodiments of the present invention are capable of removing tissue in a lateral direction.
The first important feature of the present invention is that the spacing between the electrodes and the electric field intensity therebetween is carefully controlled such that there is no direct arcing between the electrodes in the absence of tissue. The electric field intensity between the electrodes is preferably between 0.15 volts/xcexcm and 1.1 volts/xcexcm, and typically between 0.2 volts/xcexcm and 1.1 volts/xcexcm. In one preferred arrangement, the spacing between the first and second electrodes is between 0.25 mm and 1.0 mm, and the electric field intensity between the electrodes is between 0.33 volts/xcexcm and 1.1 volts/xcexcm. This ensures that the field intensity is sufficient for arcing to occur between the first electrode and the tissue, but not directly between the first and second electrodes.
However, even where direct arcing between the electrodes is prevented, there is still a potential problem if the two electrodes are similar in design. In a bipolar cutting device, only one of the electrodes will assume a high potential to tissue (and become the xe2x80x9cactivexe2x80x9d electrode), with the remaining electrode assuming virtually the same potential as the tissue (becoming the xe2x80x9creturnxe2x80x9d electrode). Where the first and second electrodes are similar, which electrode becomes the active can be a matter of circumstance. Usually, whichever electrode first contacts the tissue will become the return electrode, with the other electrode becoming the active electrode. This means that, in some circumstances, one electrode will be the active electrode, and at other times the other electrode will be the active electrode. Not only does this make the device difficult for the surgeon to control (as it will be uncertain as to exactly where the cutting action will occur), but as it is likely that any particular electrode will at some time have been active.
When an electrode is active, there is a build up of condensation products on the surface thereof. This is not a problem when the electrode continues to be the active electrode, but it does make the electrode unsuitable for use as a return electrode. Thus, in the instance where two similar electrodes are employed, it is likely that, as each will at some times become active electrode and at other times the return electrode, the build up of products on both electrodes will lead to a decrease in performance of the instrument. Therefore, the present invention provides that the first electrode has a characteristic which is dissimilar from that of the second electrode, in order to encourage one electrode to assume preferentially the role of the active electrode.
The characteristic of the first electrode which is dissimilar from that of the second electrode conveniently comprises the cross-sectional area of the electrode, the cross-sectional area of the first electrode being substantially smaller than that of the second electrode. This will help to ensure that the first electrode (being of a smaller cross-sectional area) will experience a relatively high initial impedance on contact with tissue, while the relatively larger area second electrode will experience a relatively lower initial impedance on contact with tissue. This arrangement will assist in encouraging the first electrode to become the active electrode and the second electrode to become the return electrode.
The characteristic of the first electrode which is dissimilar from that of the second electrode alternatively or additionally comprises the thermal conductivity of the electrode, the thermal conductivity of the first electrode being substantially lower than that of the second electrode. In addition to the initial impedance, the rate of rise of the impedance is a factor influencing which electrode will become active. The impedance will rise with desiccation of the tissue, and the rate of desiccation will be influenced by the temperature of the electrode. By selecting an electrode material with a relatively low thermal conductivity, the electrode temperature will rise quickly as little heat is conducted away from the part of the electrode at which the arc is forming. This will ensure a relatively fast desiccation rate, producing a correspondingly fast rise in impedance and ensuring that the first electrode remains the active electrode.
The characteristic of the first electrode which is dissimilar from that of the second electrode may further comprise the thermal capacity of the electrode, the thermal capacity of the first electrode being substantially lower than that of the second electrode. As before, a low thermal capacity helps to maintain the temperature of the first electrode at a relatively high level, ensuring that it remains the active electrode.
According to a further aspect of the invention, there is provided an electrosurgical system comprising a bipolar cutting blade, a handpiece to which the cutting blade is secured, and an electrosurgical generator for supplying a radio frequency voltage signal to the cutting blade, the cutting blade comprising first and second electrodes, and an electrical insulator spacing apart the electrodes, the spacing being between 0.25 mm and 1.0 mm, and the electrosurgical generator being adapted to supply a radio frequency voltage signal to the cutting blade which has a substantially constant peak voltage value, the peak voltage value being between 250 volts and 600 volts, the first electrode having a characteristic which is dissimilar from that of the second electrode such that the first electrode is encouraged to become an active electrode and the second electrode is encouraged to become a return electrode.
Given a particular electrode separation, it is highly desirable that the generator delivers the same peak voltages despite varying load conditions. Heavy loading of the blade may otherwise make it stall (a collapse of the arc due to too low a voltage being applied), while light loading may otherwise result in voltage overshoots and direct arcing between the electrodes.
The invention also resides in a bipolar cutting blade comprising first and second electrodes and an electrical insulator spacing apart the electrodes, the first electrode having a characteristic which is dissimilar from that of the second electrode such that the first electrode is encouraged to become an active electrode and the second electrode is encouraged to become a return electrode, the spacing between the electrodes being between 0.25 mm and 1.0 mm, such that, when the electrodes are in contact with tissue and an electrosurgical cutting voltage is applied therebetween, arcing does not occur directly between the electrodes, there also being provided means for ensuring that the temperature of the second electrode does not rise above 70xc2x0 C.
As well as ensuring that the second electrode does not become active, it is also important to ensure that the temperature of the second electrode does not rise above 70xc2x0 C., the temperature at which tissue will start to stick to the electrode. The means for ensuring that the temperature of the second electrode does not rise above 70xc2x0 C. conveniently comprises means for minimising the transfer of heat from the first electrode to the second electrode. One way of achieving this is to ensure that the first electrode is formed from a material having a relatively poor thermal conductivity, preferably less than 20 W/m.K. By making the first electrode a poor thermal conductor, heat is not transferred effectively away from the active site of the electrode and across to the second electrode, thereby helping to prevent the temperature of the second electrode from rising.
Alternatively or additionally, the heat can be inhibited from transferring from the first electrode to the second electrode by making the electrical insulator separating the electrodes from a material having a relatively poor thermal conductivity, preferably less than 40 W/m.K. Again, this helps to prevent heat generated at the first electrode from transferring to the second electrode.
Another way of inhibiting the transfer of heat is to attach the first electrode to the electrical insulator in a discontinuous manner. Preferably, the first electrode is attached to the electrical insulator at one or more point contact locations, and/or is perforated with a plurality of holes such as to reduce the percentage contact with the electrical insulator.
A preferred material for the first electrode is tantalum. When tantalum is used for the active electrode, it quickly becomes coated with a layer of oxide material. This tantalum oxide is a poor electrical conductor, helping to ensure that the first electrode maintains its high impedance with respect to the tissue, and remains the active electrode.
Another way of helping to ensure that the temperature of the second electrode does not rise above 70xc2x0 C. is to maximise the transfer of heat away from the second electrode. Thus any heat reaching the second electrode from the first electrode is quickly transferred away before the temperature of the second electrode rises inordinately. One way of achieving this is to form the second electrode from a material having a relatively high thermal conductivity, preferably greater than 150 W/m.K.
The second electrode may conveniently be provided with additional cooling means to remove heat therefrom, such as a heat pipe attached to the second electrode, or a cooling fluid constrained to flow along a pathway in contact with the second electrode. Whichever method is employed, it is advisable for there to be a temperature differential, in use, between the first and second electrodes of at least 50xc2x0 C., and preferably of between 100 and 200xc2x0 C.
Preferably, there is additionally provided a third electrode adapted to coagulate tissue. This coagulation electrode is conveniently attached to the second electrode with a further electrical insulator therebetween. It is necessary to ensure that the temperature of the coagulation electrode does not rise to too high a level, and so, if the coagulation electrode is attached to the second electrode (which is designed in accordance with the present teaching to be a good thermal conductor), it is preferable to arrange that heat is easily transferred across the further electrical insulator. This can be achieved by making the further insulator from a material having a relatively high thermal conductivity, or more typically, if the further insulator is not a good thermal conductor, by ensuring that the further insulator is relatively thin, typically no more than around 50 xcexcm. In this way the transfer of heat across the further electrical insulator is greater than 5 mW/mm2.K.
In one arrangement, the second and third electrodes are formed as conductive electrodes on an insulating substrate. Thus, both the second and third electrodes act as return electrodes when the blade is used to cut tissue with the first electrode. When the blade is used to coagulate tissue, a coagulating RF signal is applied between the second and third electrodes.
According to a final aspect of the invention, there is provided a method of cutting tissue at a target site comprising providing a bipolar cutting blade comprising first and second electrodes and an electrical insulator spacing apart the electrodes, the first electrode having a characteristic which is dissimilar from that of the second electrode such that the first electrode is encouraged to become an active electrode and the second electrode is encouraged to become a return electrode; bringing the blade into position with respect to the target site such that the second electrode is in contact with tissue at the target site and the first electrode is adjacent thereto; supplying an electrosurgical cutting voltage to the cutting blade, the electrosurgical voltage and the spacing between the first and second electrodes being such that arcing does not occur in air between the first and second electrodes, but that arcing does occur between the first electrode and the tissue at the target site, current flowing through the tissue to the second electrode; and preventing heat build up at the second electrode such that the temperature of the second electrode does not rise above 70xc2x0 C.
Preferably, the method is such that both the first and second electrodes come into contact with tissue at the target site substantially simultaneously.